In conventional shaping processes for producing white ware and advanced ceramics, a slurry is first prepared by adding an appropriate amount of water and other ingredients to a desired raw mineral material such as one for white ware or advanced ceramics to impart fluidity to the raw material and grinding and blunging the raw material with, e.g., a ball mill.
The thus-prepared slurry, having a water content of from 15 to 35%, is formed into a shape. Examples of shaping methods include the casting in which the slurry is poured into a water-absorbing mold, e.g., a gypsum mold, to allow it to be casted on the inner surface of the mold as a result of water absorption by the mold, and the resulting cast is separated from the mold; and the plastic shaping in which the slurry is dewatered on a filter press or the like to produce a press cake having a water content of from 10 to 30%, and this cake is pugged by means of a vacuum pugmill to prepare a pug for plastic shaping and then formed into a desired shape (this plastic shaping include extrusion and jiggering).
This green ware is then subjected to steps such as drying, biscuit firing, glazing, firing, and cutting to yield the final product.
In the case of the casting, the casted green ware itself is required not only to have a mechanical strength sufficient to withstand finishing and handling including conveying but also to be easily dewatered. In the case of the plastic shaping, the slurry should be dewatered without any problem, the pug should be free from air bubble inclusion, and the green ware is required to dry quickly and have a mechanical strength sufficient to withstand finishing and handling including conveying.
A technique of incorporating plastic clay, or the like into raw mineral materials in white ware production has hence been conventionally employed for imparting the plasticity necessary for plastic shaping and for also imparting mechanical strength. However, such conventional raw mineral materials are defective in that it is difficult to produce thin-walled large green wares therefrom because of the insufficient strength of the green wares.
In the production of advanced ceramics also, the shaping of green wares having sufficient strength only from raw mineral materials for advanced ceramics has been virtually impossible.
To overcome the above-described problem, a technique of incorporating a water-soluble resin, an aqueous resin emulsion, or the like at the time of slurry preparation or pugging has been added (see JP-B-3-64465, JP-B-4-32155, and JP-A-5-104509). (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively.)
Although the above prior art technique is effective in enhancing the strength of green wares due to the incorporation of a water-soluble resin or aqueous resin emulsion, it has the following drawbacks.
(i) From the standpoint of mixing, the aqueous resin emulsion or water-soluble resin should be added after the raw mineral material for white ware or advanced ceramics and other ingredients have been slurried. This mixing procedure results in foaming, which tends to reduce the mechanical strength of green wares. PA1 (ii) It is difficult to prepare a homogeneous slurry because the resin ingredient is apt to separate out due to the large difference in density between the aqueous resin emulsion or the like and the raw mineral material for white ware or advanced ceramics. Green wares obtained from such an inhomogeneous slurry tend to suffer warpage or cracking. In particular, homogeneous mixing is difficult in the case of high-viscosity slurries. PA1 (iii) The resin ingredient of the aqueous resin emulsion or the like tends to migrate to the surface of the green ware to cause tackiness. As a result, casted green wares particularly in the form of a wide flat plate or the like have impaired suitability for surface finishing. Moreover, in the forced drying of such green wares, the resin ingredient which has migrated to the green wire surface forms a film upon surface drying to cause delayed internal drying, which tends to result in warpage, cracking, blistering, etc. PA1 (iv) In the casting, the aqueous resin emulsion or water-soluble resin partly penetrates into the mold itself as a result of the dewatering of the slurry which has been poured into the mold, making demolding difficult. PA1 (v) During dewatering in plastic shaping, the water-soluble resin or aqueous resin emulsion partly migrates to the outside of the system along with the water being removed. As a result, the resin ingredient adheres to the filter cloth to cause clogging, dries and solidifies on the filter cloth surface, and causes troubles including adhesion of the filter cake to the filter cloth. Thus, the filtration for concentration becomes impossible.